Fuel cells conduct an electrochemical reaction to produce electrical power. The typical fuel cell reactants are a fuel source such as hydrogen or a hydrocarbon, and an oxidant such as air. Fuel cells provide a DC (direct current) that may be used to power motors, lights, or any number of electrical appliances. There are several different types of fuel cells, each using a different chemistry.
Fuel cells typically include three basic elements: an anode, a cathode, and an electrolyte. Usually the anode and cathode are sandwiched around the electrolyte. The electrolyte prohibits the passage of electrons. Fuel cells are usually classified by the type of electrolyte used. The fuel cell types are generally categorized into one of five groups: proton exchange membrane (PEM) fuel cells, alkaline fuel cells (AFC), phosphoric-acid fuel cells (PAFC), solid oxide fuel cells (SOFC), and molten carbonate fuel cells (MCFC).
The anode and cathode are porous and usually include an electrocatalyst, although each may have a different chemistry. Fuel migrates through the porous anode and an oxidant migrates through the porous cathode. The fuel and oxidant react to produce various charged particles, which include electrons at the anode. The electrons cannot pass through the electrolyte and therefore become an electrical current that can be directed to an external circuit. The cathode conducts the electrons back from the external circuit, where they recombine with various ions and oxygen and may form water and/or other by-products. Often a number of fuel cells are arranged in a stack to provide a useful amount of electrical power.
In many fuel cell applications, supplies of fuel and oxidant are connected to a housing that contains the fuel cell. However, much of the fuel provided to the fuel cell is often underutilized. As fuel is provided to the anode of a fuel cell, the fuel available at the surface of the anode is usually consumed quickly, while fuel at some distance from the anode is consumed more slowly and must migrate toward the anode for more efficient consumption. This phenomenon results in a fuel concentration gradient within the fuel cell. The effective use of the fuel then depends on the gas diffusion rate with which the fuel migrates to reach the anode.
Currently, fuel concentration gradients and gas diffusion rates are significant inhibitors to fuel cell performance, especially so with solid oxide fuel cells that can operate using a variety of fuels. There have been some attempts to improve fuel cell performance by quickly providing fresh supplies of fuel to the surface of the anode to fully utilize the ability of the anode to consume fuel. However, this is currently done at the expense of exhausting much of the fuel unused through the system or using a complicated manifold system with significant pressure losses.